Wireless power transmitter and method of controlling power thereof

ABSTRACT

Disclosed are a wireless power transmitter and a method of controlling power thereof. A wireless power transmitter includes a power supply device to supply AC power to the wireless power transmitter; and a transmission coil to transmit the AC power to a reception coil of a wireless power receiver by resonance. The wireless power transmitter controls transmission power to be transmitted to the wireless power receiver based on a coupling state between the transmission coil and the reception coil.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. §119 of KoreanPatent Application No. 10-2012-0107450, filed Sep. 26, 2012, which ishereby incorporated by reference in its entirety.

BACKGROUND

The embodiment relates to a wireless power transmission technology. Moreparticularly, the disclosure relates to a method of controllingtransmission power depending on the coupling states between a wirelesspower transmitter and a wireless power receiver to maximize the powertransmission efficiency.

A wireless power transmission or a wireless energy transfer refers to atechnology of wirelessly transferring electric energy to desireddevices. In the 1800's, an electric motor or a transformer employing theprinciple of electromagnetic induction has been extensively used andthen a method for transmitting electrical energy by irradiatingelectromagnetic waves, such as radio waves or lasers, has beensuggested. Actually, electrical toothbrushes or electrical razors, whichare frequently used in daily life, are charged based on the principle ofelectromagnetic induction. The electromagnetic induction refers to aphenomenon in which voltage is induced so that current flows when amagnetic field is varied around a conductor. Although thecommercialization of the electromagnetic induction technology has beenrapidly progressed around small-size devices, the power transmissiondistance thereof is short.

Until now, wireless energy transmission schemes include a remotetelecommunication technology based on magnetic resonance and a shortwave radio frequency in addition to the electromagnetic induction.

Recently, among wireless power transmitting technologies, an energytransmitting scheme employing resonance has been widely used.

However, according to the energy transmitting scheme employing resonanceaccording to the related art, the power transmission efficiency may bevaried depending on the coupling states between the wireless powertransmitter and the wireless power receiver.

Therefore, a scheme of maximizing the power transmission efficiency byreflecting the coupling state between the wireless power transmitter andthe wireless power receiver is required.

BRIEF SUMMARY

The embodiment provides a method of maximizing the power transmissionefficiency depending on the coupling state between a wireless powertransmitter and a wireless power receiver.

The embodiment provides a method of controlling the transmission powerdepending on a coupling coefficient between a wireless power transmitterand a wireless power receiver by detecting the coupling coefficientbetween the wireless power transmitter and the wireless power receiver.

According to one embodiment, there is provided a wireless powertransmitter to transmit power to a load through a wireless powerreceiver. The wireless power transmitter includes a power supply unit tosupply AC power to the wireless power transmitter, and a transmissioncoil to transmit the AC power to a reception coil of a wireless powerreceiver by resonance. The wireless power transmitter controlstransmission power to be transmitted to the wireless power receiverbased on a coupling state between the transmission coil and thereception coil.

According to one embodiment, a method of controlling power of a wirelesspower transmitter to transmit the power to a load through a wirelesspower receiver includes detecting a coupling state between the wirelesspower transmitter and the wireless power receiver, adjustingtransmission power based on the coupling state, and transmitting theadjusted transmission power to the load by resonance.

As described above, there can be provided a method of maximizing thepower transmission efficiency by controlling transmission poweraccording to the coupling state between the wireless power transmitterand the wireless power receiver.

According to the embodiment, the coupling coefficient between thewireless power transmitter and the wireless power receiver is detectedand the optimal reception power is determined based on the couplingcoefficient. The power transmission efficiency can be maximized bycontrolling the transmission power according to the determined receptionpower.

Meanwhile, any other various effects will be directly and implicitlydescribed below in the description of the embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the structure of a wireless powertransmission system according to one embodiment.

FIG. 2 is an equivalent circuit diagram showing the wireless powertransmission system according to one embodiment.

FIG. 3 is a flowchart to explain a method of controlling power accordingto one embodiment.

FIG. 4 is a graph showing the relation between a coupling coefficientand a load impedance in order to satisfy the maximum power transmissionefficiency.

FIG. 5 is a graph showing an example of the relation between thecoupling coefficient and the load impedance in order to satisfy themaximum power transmission efficiency when a load is a battery.

FIG. 6 is a graph showing relation between current and voltage appliedto a battery when a load is the battery.

FIG. 7 is a graph showing the relation between the quantity of powerapplied to a battery and load impedance when the load is the battery.

FIG. 8 is a graph showing the relation between the coupling coefficientand the load in order to satisfy the maximum power transmissionefficiency when the load is the battery.

FIG. 9 is a block diagram showing the structure of a wireless powertransmission system according to another embodiment.

FIG. 10 is a ladder diagram to explain a method of controlling poweraccording to another embodiment.

FIG. 11 is a flowchart to explain a method of controlling poweraccording to another embodiment.

FIG. 12 is a view to explain a look-up table in which a current valuewhen a first output voltage is applied to an AC power generating unit, acoupling coefficient, a second output voltage, and a preferable currentrange correspond to each other.

FIG. 13 is a flowchart to explain a method of detecting a couplingcoefficient according to another embodiment.

FIG. 14 is a view to explain the case that a switch is open in order tochange output impedance.

FIG. 15 is a view to explain the case that the switch is shorted inorder to change the output impedance.

FIG. 16 is a flowchart to explain a method of controlling poweraccording to still another embodiment.

FIG. 17 is a view to explain a look-up table used in the method ofcontrolling power according to the embodiment of FIG. 16.

DETAILED DESCRIPTION

Hereinafter, embodiments will be described in detail with reference toaccompanying drawings so that those skilled in the art can easily workwith the embodiments.

According to the present invention, a scheme of transmitting powerthrough electromagnetic induction may signify a tightly coupling schemehaving a relatively low Q value, and a scheme of transmitting powerthrough resonance may signify a loosely coupling scheme having arelatively high Q value.

According to one embodiment, the frequency band used for powertransmission in the tightly coupling scheme may be in the range of 100kHz to 300 kHz, and the frequency band used for power transmission inthe loosely coupling scheme may be one of 6.78 MHz and 13.56 MHz.However, the above numeric values are provided for the illustrativepurpose.

In addition, the loosely coupling scheme of transmitting power throughresonance according to the embodiment may include a directly couplingscheme and an inductively coupling scheme.

According to the directly coupling scheme, each of a wireless powertransmitter and a wireless power receiver directly performs powertransmission by using one resonant coil. According to the inductivelycoupling scheme, a wireless power transmitter transmits power to awireless power receiver including two reception coils through twotransmission coils.

FIG. 1 is a block diagram showing the structure of a wireless powertransmission system 10 according to one embodiment, and FIG. 2 is anequivalent circuit diagram showing the wireless power transmissionsystem 10 according to one embodiment.

Referring to FIG. 1, the wireless power transmission system 10 mayinclude a power supply device 100, a wireless power transmitter 200, awireless power receiver 300, and a load 400.

According to one embodiment, the power supply device 100 may be providedseparately from the wireless power transmitter 200 as shown in FIG. 1 ormay be included in the wireless power transmitter 200.

Referring to FIG. 1, the power supply device 100 may include a powersupply unit 110, a switch 120, a DC-DC converter 130, a powertransmission state detecting unit 140, an oscillator 150, an AC powergenerating unit 160, a control unit 180, and a storage unit 170.

The power supply unit 110 may supply DC power to each component of thepower supply device 100. The power supply unit 110 may be providedseparately from the power supply device 100.

According to one embodiment, the wireless power transmitter 200 maytransmit power to the wireless power receiver 300 by using resonance.The transmission coil of the wireless power transmitter 200 may berealized based on the inductively coupling scheme by including atransmission induction coil unit 211 and a transmission resonant coilunit 212 to be described later, or may be realized based on the directlycoupling scheme by including only one transmission induction coil unit211. The switch 120 may connect the power supply unit 110 with the DC-DCconverter 130, or disconnect the power supply unit 110 from the DC-DCconverter 130. The switch 120 may be open or shorted by an open signalor a short signal of the control unit 180. According to one embodiment,the switch 120 may be open or shorted by the operation of the controlunit 180 according to the power transmission state between the wirelesspower transmitter 200 and the wireless power receiver 300.

The DC-DC converter 130 may convert DC voltage, which is received fromthe power supply unit 110, into DC voltage having a predeterminedvoltage value to be output.

After converting the DC voltage received from the power supply unit 110into AC voltage, the DC-DC converter 130 may boost up or drop down andrectify the converted AC voltage, and output the DC voltage having apredetermined voltage value.

The DC-DC converter 130 may include a switching regulator or a linearregulator.

The linear regulator is a converter to receive input voltage to output arequired quantity of voltage and to discharge the remaining quantity ofvoltage as heat.

The switching regulator is a converter capable of adjusting outputvoltage through a pulse width modulation (PWM) scheme.

The power transmission state detecting unit 140 may detect the powertransmission state between the wireless power transmitter 200 and thewireless power receiver 300. According to one embodiment, the powertransmission state detecting unit 140 may detect the coupling statebetween the wireless power transmitter 200 and the wireless powerreceiver 300 by detecting the power transmission state. In this case,the coupling state may represent at least one of the distance betweenthe wireless power transmitter 200 and the wireless power receiver 300and the positions of the wireless power transmitter 200 and the wirelesspower receiver 300.

According to one embodiment, the power transmission state detecting unit140 may detect the power transmission state based on current flowing inthe power supply device 100. To this end, the power transmission statedetecting unit 140 may include a current sensor. The current sensor maymeasure current flowing in the power supply device 100, and may detectthe coupling state between the wireless power transmitter 200 and thewireless power receiver based on the current. The coupling state may beexpressed as a coupling coefficient between the transmission resonantcoil unit 212 of the wireless power transmitter 200 and a receptionresonant coil unit 311 of the wireless power receiver 300.

According to one embodiment, the power transmission state detecting unit140 may measure the intensity of current flowing when the DC voltageoutput from the DC-DC converter 130 is applied to the AC powergenerating unit 160, but the embodiment is not limited thereto. In otherwords, the power transmission state detecting unit 140 may measure theintensity of current output from the AC power generating unit 160.

According to one embodiment, the power transmission state detecting unit140 may include a current transformer (CT). According to one embodiment,the intensity of current applied to the AC power generating unit may beused to find the distance between the wireless power transmitter 200 andthe wireless power receiver 300. According to one embodiment, theintensity of the current applied to the AC power generating unit 160 mayserve as an index to represent the coupling state between the wirelesspower transmitter 200 and the wireless power receiver 300. The powertransmission state detecting unit 140 may transmit a signal representingthe intensity of the detected current to the control unit 180.

Although FIG. 1 shows that the power transmission state detecting unit140 is provided separately from the control unit 180, the powertransmission state detecting unit 140 may be included in the controlunit 180.

The oscillator 150 may generate an AC signal having a predeterminedfrequency and apply the AC signal to the AC power generating unit 160.

The AC power generating unit 160 may generate AC power by using the DCvoltage received from the DC-DC converter 130 and the AC signal.

The AC power generating unit 160 may amplify the AC signal generatedfrom the oscillator 150. An amount of an AC signal to be amplified maybe varied depending on the intensity of the DC voltage through the DC-DCconverter 130.

According to one embodiment, the AC power generating unit 160 mayinclude a push-pull type dual MOSFET.

The control unit 180 may control the overall operation of the powersupply device 100.

The control unit 180 may control the DC-DC converter 130 so that presetDC voltage is applied to the AC power generating unit 160.

The control unit 180 may receive a signal, which is related to theintensity of current flowing when the DC voltage output from the DC-DCconverter 130 is applied to the AC power generating unit 160, from thepower transmission state detecting unit 140, and adjust at least one ofthe DC voltage output from the DC-DC converter 130 and the frequency ofthe AC signal output from the oscillator 150 by using the signal relatedto the intensity of the received current.

The control unit 180 receives the signal representing the intensity ofthe current applied to the AC power generating unit 160 from the powertransmission state detecting unit 140 to determine if the wireless powerreceiver 300 exists. In other words, the control unit 180 may determinethe existence of the wireless power receiver 300 capable of receivepower from the wireless power transmitter 200 based on the intensity ofthe current applied to the AC power generating unit 160.

The control unit 180 may control the oscillator 150 to generate an ACsignal having a predetermined frequency. The predetermined frequency mayrefer to a resonance frequency of the wireless power transmitter 200 andthe wireless power receiver 300 when the power transmission is performedby using resonance.

The storage unit 170 may store the intensity of the current applied tothe AC power generating unit 160, the coupling coefficient between thewireless power transmitter 200 and the wireless power receiver 300, andthe DC voltage output from the DC-DC converter 130 corresponding to eachother. In other words, the storage unit 170 may store three values inthe form of a look-up table.

The control unit 180 may search for a coupling coefficient correspondingto the intensity of the current applied to the AC power generating unit160 and DC voltage output from the DC-DC converter 130 in the storageunit 170, and may control the DC-DC converter 130 so that the searchedDC voltage may be output.

The wireless power transmitter 200 receives AC power from the AC powergenerating unit 160.

When the wireless power transmitter 200 is realized based on theinductively coupling scheme, the wireless power transmitter 200 mayinclude the transmission induction coil unit 211 and the transmissionresonant coil unit 212 constituting a transmission unit 210 shown inFIG. 2 to be described later.

When the wireless power transmitter 200 is realized based on thedirectly coupling scheme, the wireless power transmitter 200 may includeonly the transmission induction coil unit 211 among components of thetransmission unit 210 shown in FIG. 2 to be described later.

The transmission resonant coil unit 212 may transmit the AC powerreceived from the transmission induction coil unit 211 to the wirelesspower receiver 300 by using resonance. In this case, the wireless powerreceiver 300 may include the reception resonant coil L₃ and thereception induction coil L₄ shown in FIG. 2.

Referring to FIG. 2, the wireless power transmission system 10 mayinclude a power supply device 100, the wireless power transmitter 200,the wireless power receiver 300, and the load 400.

The power supply device 100 includes all components described withreference to FIG. 1, and the components basically include the functionsdescribed with reference to FIG. 1.

The wireless power transmitter 200 may include the transmission unit 210and a detection unit 220.

The transmission unit 210 may include the transmission induction coilunit 211 and the transmission resonant coil unit 212.

The AC power generated from the power supply device 100 is transmittedto the wireless power transmitter 200, and transmitted to the wirelesspower receiver 300 making resonance together with the wireless powertransmitter 200. The power received in the wireless power receiver 300is transmitted to the load 400 through a rectifying unit 320.

The load 400 may signify a rechargeable battery or other predetermineddevices requiring power. According to the embodiment, the load impedanceof the load 400 may be expressed as “R_(L)”. According to oneembodiment, the load 400 may be included in the wireless power receiver300.

The power supply device 100 may supply AC power having a predeterminedfrequency to the wireless power transmitter 200. The power supply device100 may supply AC power having a resonance frequency in resonancebetween the wireless power transmitter 200 and the wireless powerreceiver 300.

The transmission unit 210 may include the transmission induction coilunit 211 and the transmission resonant coil unit 212.

The transmission induction coil unit 211 is connected to the powersupply device 100, and AC current flows through the transmissioninduction coil unit 211 by power received from the power supply device100. If the AC current is flows through the transmission induction coilunit 211, the AC current is induced even to the transmission resonantcoil unit 212 physically spaced apart from the transmission inductioncoil unit 211 due to electromagnetic induction. The power induced to thetransmission resonant coil unit 212 is transmitted to the wireless powerreceiver 300 forming a resonant circuit together with the wireless powertransmitter 200 through the resonance.

Power can be transmitted between two LC circuits, which areimpedance-matched with each other, through resonance. Since thetransmission resonant coil unit 212 is loosely coupled with thereception resonant coil unit 311, the power transmitted through theresonance can be farther transmitted when comparing with the powertransmitted in the case of the tightly coupling scheme through theelectromagnetic induction. Accordingly, the wireless power transmitter200 and the wireless power receiver 300 have the higher alignment freedegree so that the wireless power transmitter 200 and the wireless powerreceiver 300 transmit power with higher efficiency.

The transmission resonant coil unit 212 of the wireless powertransmitter 200 may transmit power to the reception resonant coil unit311 of the wireless power receiver 300 through a magnetic field.

In detail, the transmission resonant coil unit 212 and the receptionresonant coil unit 311 are magnetically loosely coupled with each other.

Since the transmission resonant coil unit 212 is loosely coupled withthe reception resonant coil unit 311, the power transmission efficiencybetween the wireless power transmitter 200 and the wireless powerreceiver 300 can be significantly improved.

The transmission induction coil unit 211 may include a transmissioninduction coil L₁ and a capacitor C₁. In this case, the capacitance ofthe capacitor C₁ is a value adjusted for the operation at the resonancefrequency.

One terminal of the capacitor C₁ is connected to one terminal of thepower supply device 100, and an opposite terminal of the capacitor C₁ isconnected to one terminal of the transmission induction coil L₁. Anopposite terminal of the transmission induction coil L₁ is connected toan opposite terminal of the power supply device 100.

The transmission resonant coil unit 212 includes a transmission resonantcoil L₂, a capacitor C₂, and a resistor R₂. The transmission resonantcoil L₂ includes one terminal connected to one terminal of the capacitorC₂ and an opposite terminal connected to one terminal of the resistorR₂. The opposite terminal of the resistor R₂ is connected to theopposite terminal of the capacitor C₂. The resistance of the resistor R₂represents the quantity of power loss in the transmission resonant coilL₂, and the capacitance of the capacitor C₂ is a value adjusted for theoperation at the resonance frequency.

The detection unit 220 may detect the coupling state between thewireless power transmitter 200 and the wireless power receiver 300.According to one embodiment, the coupling state may be detected based onthe coupling coefficient between the transmission resonant coil unit 212and the reception resonant coil unit 311. In this case, the detectionunit 220 may detect the coupling coefficient by measuring the inputimpedance, and the detail thereof will be described later.

The wireless power receiver 300 may include a reception unit 310 and arectifying unit 320.

The wireless power receiver 300 may be embedded in an electronicappliance such as a cellular phone, a mouse, a laptop computer, and anMP3 player.

The reception unit 310 may include a reception resonant coil unit 311and a reception induction coil unit 312.

The reception resonant coil unit 311 includes a reception resonant coilL₃, a capacitor C₃, and a resistor R₃. The reception resonant coil L₃includes one terminal connected to one terminal of the capacitor C₃ andan opposite terminal connected to one terminal of the resistor R₃. Anopposite terminal of the resistor R₃ is connected to an oppositeterminal of the capacitor C₃. The resistance of the resistor R₃represents the quantity of power loss in the transmission resonant coilL₃, and the capacitance of the capacitor C₃ is a value adjusted for theoperation at the resonance frequency.

The reception induction coil unit 312 includes a reception inductioncoil L₄ and a capacitor C₄. The reception resonant coil L₄ includes oneterminal connected to one terminal of the capacitor C₄. An oppositeterminal of the reception induction coil L₄ is connected to an oppositeterminal of the rectifying unit 320. An opposite terminal of thecapacitor C₄ is connected to one terminal of the rectifying unit 320.

The reception resonant coil unit 311 and the transmission resonant coilunit 212 maintain a resonance state at a resonance frequency. In otherwords, the reception resonant coil unit 311 and the transmissionresonant coil unit 212 are resonance-coupled with each other so that ACcurrent flows through the reception resonant coil unit 311. Accordingly,the reception resonant coil unit 311 may receive power from the wirelesspower transmitter 200 through a non-radiative scheme.

The reception induction coil unit 312 receives power from the receptionresonant coil unit 311 through the electromagnetic induction, and thepower received in the reception induction coil unit 312 is rectified bythe rectifying unit 320 and sent to the load 400.

The rectifying unit 320 may receive the AC power from the receptioninduction coil unit 312 and convert the received AC power into DC power.

The rectifying unit 320 may include a rectifying circuit (not shown) anda smoothing circuit (not shown).

The rectifying circuit may include a diode and a capacitor to convertthe AC power received from the reception induction coil unit 312 to DCpower and sent the DC power to the load 400.

The smoothing circuit may smooth the rectified output. The smoothingcircuit may include a capacitor.

The load 400 may receive the DC power rectified from the rectifying unit320.

The load 400 may be a predetermined rechargeable battery or devicerequiring the DC power. For example, the load 400 may refer to a batteryof a cellular phone, but the embodiment is not limited thereto.

According to one embodiment, the load 400 may be included in thewireless power receiver 300.

A quality factor and a coupling coefficient are important in thewireless power transmission.

The quality factor may refer to an index of energy that may be stored inthe vicinity of the wireless power transmitter or the wireless powerreceiver.

The quality factor may be varied depending on the operating frequency was well as a shape, a dimension and a material of a coil. The qualityfactor may be expressed as following equation, Q=ω*L/R. In the aboveequation, L refers to the inductance of a coil and R refers toresistance corresponding to the quantity of power loss caused in thecoil.

The quality factor may have a value of 0 to infinity.

The coupling coefficient represents the degree of magnetic couplingbetween a transmission coil and a reception coil, and has a value in therange of 0 to 1.

The coupling coefficient may be varied depending on the relativeposition and distance between the transmission coil and the receptioncoil.

The wireless power transmitter 200 may interchange information with thewireless power receiver 300 through in-band communication or out-of-bandcommunication.

The in-band communication refers to the communication for interchanginginformation between the wireless power transmitter 200 and the wirelesspower receiver 300 through a signal having the frequency used in thewireless power transmission. The wireless power receiver 300 may furtherinclude a switch and may receive or may not receive power transmittedfrom the wireless power transmitter 200 through a switching operation ofthe switch. Accordingly, the wireless power transmitter 200 canrecognize an on-signal or an off-signal of the wireless power receiver300 by detecting the quantity of power consumed in the wireless powertransmitter 200.

In detail, the wireless power receiver 300 may change the power consumedin the wireless power transmitter 200 by adjusting the quantity of powerabsorbed in a resistor by using the resistor and the switch. Thewireless power transmitter 200 may acquire the state information of thewireless power receiver 300 by detecting the variation of the powerconsumption. The switch may be connected to the resistor in series.According to one embodiment, the state information of the wireless powerreceiver 300 may include information about the present charge quantityand the change of the charge quantity in the wireless power receiver300.

In more detail, if the switch is open, the power absorbed in theresistor becomes zero, and the power consumed in the wireless powertransmitter 200 is reduced.

If the switch is short-circuited, the power absorbed in the resistorbecomes greater than zero, and the power consumed in the wireless powertransmitter 200 is increased. If the wireless power receiver repeats theabove operation, the wireless power transmitter 200 detects powerconsumed therein to make digital communication with the wireless powerreceiver 300.

The wireless power transmitter 200 receives the state information of thewireless power receiver 300 through the above operation so that thewireless power transmitter 200 can transmit appropriate power.

To the contrary, the wireless power transmitter 200 may include aresistor and a switch to transmit the state information of the wirelesspower transmitter 200 to the wireless power receiver 300. According toone embodiment, the state information of the wireless power transmitter200 may include information about the maximum quantity of power to besupplied from the wireless power transmitter 200, the number of wirelesspower receivers 300 receiving the power from the wireless powertransmitter 200 and the quantity of available power of the wirelesspower transmitter 200.

The out-of-band communication refers to the communication performedthrough a specific frequency band other than the resonance frequencyband in order to exchange information necessary for the powertransmission. The wireless power transmitter 200 and the wireless powerreceiver 300 can be equipped with out-of-band communication modules toexchange information necessary for the power transmission. Theout-of-band communication module may be installed in the power supplydevice. In one embodiment, the out-of-band communication module may usea short-distance communication technology, such as Bluetooth, Zigbee,WLAN or NFC, but the embodiment is not limited thereto.

Hereinafter, a method of controlling power according to one embodimentwill be described in detail with reference to FIGS. 3 to 8.

FIG. 3 is a flowchart to explain the method of controlling poweraccording to one embodiment. FIG. 4 is a graph showing the relationbetween a coupling coefficient and a load impedance in order to satisfythe maximum power transmission efficiency. FIG. 5 is a graph showing anexample of the relation between the coupling coefficient and the loadimpedance in order to satisfy the maximum power transmission efficiencywhen a load is a battery. FIG. 6 is a graph showing relation betweencurrent and voltage applied to a battery when a load is the battery.FIG. 7 is a graph showing the relation between the quantity of powerapplied to a battery and load impedance when the load is the battery.FIG. 8 is a graph showing the relation between the coupling coefficientand the load in order to satisfy the maximum power transmissionefficiency when the load is the battery.

Hereinafter, the method of controlling the power will be described withreference to FIG. 3 as well as FIGS. 1 and 2.

The wireless power transmitter 200 measures an input impedance (stepS101). The input impedance may be a first input impedance Z₁. The firstinput impedance Z₁ may be impedance when viewed from the power supplydevice 100 to the wireless power transmitter 200 as shown in FIG. 2.According to one embodiment, the detection unit 220 may measure thefirst input impedance Z₁ by using current and voltage input to thewireless power transmitter 200 from the power supply device 100.

Referring to FIG. 3 again, the detection unit 220 detects the couplingstate between the wireless power transmitter 200 and the wireless powerreceiver 300 by using the input impedance (step S103). According to oneembodiment, the coupling state between the wireless power transmitter200 and the wireless power receiver 300 may be detected by measuring acoupling coefficient K₂ between the transmission resonant coil L₂ andthe reception resonant coil L₃. In this case, the coupling coefficientK₂ represents the electromagnetic coupling degree between thetransmission resonant coil L₂ and the reception resonant coil L₃. Thecoupling coefficient K₂ may be varied depending on at least one of thedistance between the wireless power transmitter 200 and the wirelesspower receiver 300, and the directions and the positions of the wirelesspower transmitter 200 and the wireless power receiver.

The detected coupling state may be used to control the power to betransmitted to the wireless power receiver 300 by the wireless powertransmitter 200. According to one embodiment, the wireless powertransmitter 200 may increase the quantity of the power to be transmittedto the wireless power receiver 300 as the magnetic coupling between thewireless power transmitter 200 and the wireless power receiver 300 isweakened, and may decrease the quantity of the power to be transmittedto the wireless power receiver 300 as the magnetic coupling between thewireless power receiver 200 and the wireless power receiver 300 isstrengthened.

Hereinafter, the method of detecting the coupling state, particularly,the coupling coefficient will be described.

Referring to FIG. 2, a third input impedance Z₃ may refer to animpedance when viewed from the reception resonant coil unit 311 to thereception induction coil unit 312, and may be expressed as Equation 1.

$\begin{matrix}{Z_{3} = \frac{\omega^{2}M_{3}^{2}}{Z_{L} + {{j\omega}\; L_{4}} + \frac{1}{{j\omega}\; C_{4}}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

In Equation 1, ω represents the resonance frequency when thetransmission resonant coil L₂ and a reception resonant coil L₃ makeresonance, and M₃ refers to the mutual inductance between the receptionresonant coil L₃ and the reception induction coil L₄. In addition, Z_(L)refers to an output impedance. The output impedance Z_(L) may be equalto the impedance R_(L) of the load 400.

The mutual inductance M₃ may be calculated through Equation 2.

M ₃ =K ₃√{square root over (L ₃ L ₄)}  Equation 2

In Equation 2, K₃ represents the coupling coefficient between thereception resonant coil L₃ and the reception induction coil L₄ and is afixed value. Since the inductance of the reception resonant coil L₃ andthe inductance of the reception induction coil L₄ are fixed values, themutual inductance M₃ is a fixed value.

Since the resonance frequency w, the mutual inductance M₃, the loadimpedance Z_(L), the inductance of the reception induction coil L₄, andthe capacitance of the capacitor C₄ are fixed values, the third inputimpedance Z₃ has a fixed value.

Equation 1 is expressed based on a frequency domain, and followingequations are expressed based on frequency domains.

The second input impedance Z₂ refers to an impedance when viewed fromthe wireless power transmitter 200 to the wireless power receiver 300,and may be expressed as Equation 3.

$\begin{matrix}{Z_{2} = {\frac{{j\omega}^{3}C_{3}M_{2}^{2}}{1 - {\omega^{2}L_{3}C_{3}} + {{j\omega}\; {C_{3}\left( {Z_{3} + R_{3}} \right)}}}.}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

In Equation 3, M₂ refers to the mutual inductance between thetransmission resonant coil L₂ and the reception resonant coil L₃, and C₃refers to a capacitor expressed when the reception resonant coil unit311 is converted into an equivalent circuit. In addition, R3 representsthe quantity of power loss occurring in the reception resonant coil L3as a resistance.

The capacitance of the capacitor C₃, the inductance of the receptionresonant coil L3, the third input impedance Z₃, and the resistor R₃ arefixed values.

The mutual inductance M₂ may be calculated through Equation 4.

M ₂ =K ₂√{square root over (L ₂ L ₃)}  Equation 4

In Equation 4, since the inductance of the transmission resonant coil L₂and the inductance of the reception resonant coil L₃ are fixed values,the mutual inductance M₂ may be varied depending on the couplingcoefficient K₂ between the transmission resonant coil L₂ and thereception resonant coil L₃.

Accordingly, if the third input impedance Z₃ in Equation 1 issubstituted into Equation 3, the second input impedance Z₂ may beexpressed in relation to the mutual inductance M₂, and may be varieddepending on the mutual inductance M₂.

The first input impedance Z₁ refers to an impedance when viewed from thepower supply device 100 to the wireless power transmitter 200, and maybe expressed as Equation 5.

$\begin{matrix}{Z_{1} = {{{j\omega}\; L_{1}} + \frac{1}{{j\omega}\; C_{1}} + {\frac{{j\omega}^{3}C_{2}M_{1}^{2}}{1 - {\omega^{2}L_{2}C_{2}} + {{j\omega}\; {C_{2}\left( {Z_{2} + R_{2}} \right)}}}.}}} & {{Equation}\mspace{14mu} 5}\end{matrix}$

In Equation 5, M₁ refers to the mutual inductance between thetransmission induction coil L₁ and the transmission resonant coil L₂.

The mutual inductance M₁ may be calculated through Equation 6.

M ₁ =K ₁√{square root over (L ₁ L ₂)}  Equation 6

In Equation 6, since the inductance of the transmission resonant coilL₁, the inductance of the transmission induction coil L₂, and thecoupling coefficient K₁ between the transmission resonant coil L₁ andthe transmission induction coil L₂ are fixed values, the mutualinductance M₁ has a fixed value.

Although the inductance of the transmission induction coil L₁, thecapacitance of the capacitor C₁, the mutual inductance M₁, theinductance of the transmission resonant coil L₂, the capacitor C₂, andthe resistor R₂ have fixed values, the second input impedance Z₂ may bevaried depending on the mutual inductance M₂.

If Equation 2 is substituted into Equation 3, the first input impedanceZ₁ may be expressed in relation to the mutual inductance M₂.

The detection unit 220 may calculate the mutual inductance M₂ by usingthe first input impedance Z₁ in the equation for the first inputimpedance Z₁ measured in step S101 and the mutual inductance M₂, and maydetect the coupling coefficient K₂ through the calculated mutualinductance M₂ and Equation 4.

Another scheme of detecting the coupling coefficient K₂ will bedescribed with reference to FIG. 13.

Referring to FIG. 3 again, the wireless power transmitter 200 decidesreception power corresponding to the detected coupling state (stepS105). In this case, the determined reception power may refer to powerthat the load 400 must receive in order to maximize the powertransmission efficiency between the wireless power transmitter 200 andthe load 400.

Hereinafter, a scheme of detecting the coupling coefficient K₂ anddeciding the reception power that the load 400 must receive depending onthe coupling coefficient K₂ will be described.

Referring to FIG. 2, the power transmission efficiency may be calculatedthrough following Equation 7.

$\begin{matrix}{{Efficiency} = {\frac{P_{out}}{P_{in}} = \frac{I_{L}^{2}R_{L}}{I_{1}^{2}Z_{1}}}} & {{Equation}\mspace{14mu} 7}\end{matrix}$

In Equation 7, P_(in) may refer to transmission power transmitted to thewireless power transmitter 200 by the power supply device 100, andP_(out) may refer to power consumed in the load 400 and reception powerreceived in the load 400. I₁ is current flowing through the load 400.

The current I₁ is current input to the wireless power transmitter 200while serving as current flowing through the transmission induction coilunit 211.

The current I₁ may be calculated through the following procedure.

When current flowing through the reception resonant coil unit 311 isrepresented as I₃, the current I₃ may be expressed as following Equation8.

$\begin{matrix}{I_{3} = {\frac{R_{L}}{{j\omega}\; M_{3}}I_{L}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

When current flowing through the transmission resonant coil unit 212 isrepresented as I₂, the current I₂ may be expressed as following Equation9.

$\begin{matrix}{I_{2} = {\frac{I_{3}}{\omega^{2}M_{2}C_{3}}\left( {1 - {\omega^{2}L_{3}} + {{j\omega}\; {C_{3}\left( {R_{3} + Z_{3}} \right)}}} \right)}} & {{Equation}\mspace{14mu} 9}\end{matrix}$

When current flowing through the transmission induction coil unit 211 isrepresented as I₁, the current I₁ may be expressed as following Equation10.

$\begin{matrix}{I_{1} = {\frac{I_{2}}{\omega^{2}M_{1}C_{2}}\left( {1 - {\omega^{2}L_{2}} + {{j\omega}\; {C_{2}\left( {R_{2} + Z_{2}} \right)}}} \right)}} & {{Equation}\mspace{14mu} 10}\end{matrix}$

Equation 8 is substituted into Equation 9, and the substitution resultof Equation 9 is substituted into Equation 10. Next, the substitutionresult of Equation 10 and the first input impedance Z₁ represented asthe mutual inductance M₂ are substituted in Equation 7. In this case,Equation 11 is obtained in relation to the power transmissionefficiency.

$\begin{matrix}{{Efficiency} = {\left( \frac{{j\omega}\; M_{3}j\; \omega \; M_{2}{j\omega}\; M_{1}}{{R_{2}R_{3}R_{L}} + {\omega^{2}M_{3}^{2}R_{2}} + {\omega^{2}M_{2}^{2}R_{L}}} \right)^{2}\frac{{R_{2}R_{3}R_{L}} + {\omega^{2}M_{3}^{2}R_{2}} + {\omega^{2}M_{2}^{2}R_{L}}}{\omega^{2}{M_{1}^{2}\left( {{R_{3}R_{L}} + {\omega^{2}M_{3}^{2}}} \right)}}R_{L}}} & {{Equation}\mspace{14mu} 11}\end{matrix}$

Equation 11 is arranged as Equation 12.

                                     Equation  12 $\begin{matrix}{{Efficiency} = {\frac{\left( {{j\omega}\; M_{3}{j\omega}\; M_{2}{j\omega}\; M_{1}} \right)^{2}}{\left( {{R_{2}R_{3}R_{L}} + {\omega^{2}M_{3}^{2}R_{2}} + {\omega^{2}M_{2}^{2}R_{L}}} \right)\omega^{2}{M_{1}^{2}\left( {{R_{3}R_{L}} + {\omega^{2}M_{3}^{2}}} \right)}}R_{L}}} \\{= {\frac{\omega^{4}M_{3}^{2}M_{2}^{2}}{\left( {{R_{2}R_{3}R_{L}} + {\omega^{2}M_{3}^{2}R_{2}} + {\omega^{2}M_{2}^{2}R_{L}}} \right)\left( {{R_{3}R_{L}} + {\omega^{2}M_{3}^{2}}} \right)}R_{L}}}\end{matrix}$

The quality factor Q₂ of the transmission resonant coil unit 212 isexpressed as following Equation 13, and the quality factor Q₃ of thereception resonant coil unit 311 is expressed as Equation 14.

$\begin{matrix}\begin{matrix}{Q_{2} = \frac{\omega \; L_{2}}{R_{2}}} \\{= {\frac{1}{R_{2}}\sqrt{\frac{L_{2}}{C_{2}}}}}\end{matrix} & {{Equation}\mspace{14mu} 13} \\\begin{matrix}{Q_{3} = \frac{\omega \; L_{3}}{R_{3}}} \\{= {\frac{1}{R_{3}}\sqrt{\frac{L_{3}}{C_{3}}}}}\end{matrix} & {{Equation}\mspace{14mu} 14}\end{matrix}$

When Equation 13 and Equation 14 are substituted into Equation 12, thesubstitution result is arranged as following Equation 15.

$\begin{matrix}\begin{matrix}{{Efficiency} = {\frac{\omega^{4}L_{3}L_{4}L_{3}L_{2}k_{2}^{2}}{\left( {{\frac{\omega \; L_{2}}{Q_{2}}\frac{\omega \; L_{3}}{Q_{3}}R_{L}} + {\omega^{2}L_{3}L_{4}\frac{\omega \; L_{2}}{Q_{2}}} + {\omega^{2}k_{2}^{2}L_{3}L_{2}R_{L}}} \right)}R_{L}}} \\{\left( {{\frac{\omega \; L_{3}}{Q_{3}}R_{L}} + {\omega^{2}L_{3}L_{4}}} \right)} \\{= \frac{\omega \; L_{4}k_{2}^{2}}{\left( {{\frac{1}{Q_{2}}\frac{1}{Q_{3}}} + {L_{4}\frac{\omega}{R_{L}Q_{2}}} + k_{2}^{2}} \right)\left( {{\frac{1}{Q_{3}}R_{L}} + {\omega \; L_{4}}} \right)}} \\{= \frac{k_{2}^{2}Q_{2}Q_{3}}{\left( {1 + \frac{\omega \; Q_{3}L_{4}}{R_{L}} + {Q_{2}Q_{3}k_{2}^{2}}} \right)\left( {\frac{R_{L}}{Q_{3}\omega \; L_{4}} + 1} \right)}}\end{matrix} & {{Equation}\mspace{14mu} 15}\end{matrix}$

For the calculation convenience, x is substituted as Equation 16, and mis substituted as Equation 17.

$\begin{matrix}{x = \frac{\omega \; Q_{3}L_{4}}{R_{L}}} & {{Equation}\mspace{14mu} 16} \\{m = {k_{2}^{2}Q_{2}Q_{3}}} & {{Equation}\mspace{14mu} 17}\end{matrix}$

If Equation 16 and Equation 17 are substituted into Equation 15 which isan equation for power transmission efficiency, the power transmissionefficiency may be arranged as Equation 18.

$\begin{matrix}\begin{matrix}{{Efficiency} = \frac{m}{\left( {1 + x + m} \right)\left( {\frac{1}{x} + 1} \right)}} \\{= \frac{mx}{\left( {x + m + 1} \right)\left( {x + 1} \right)}} \\{= \frac{mx}{x^{2} + {\left( {m + 2} \right)x} + m + 1}}\end{matrix} & {{Equation}\mspace{14mu} 18}\end{matrix}$

If Equation 18 is differentiated with respect to x in order to obtain acondition of maximizing the power transmission efficiency, followingEquation 19 may be obtained.

$\begin{matrix}\begin{matrix}{\frac{Efficiency}{dx} = \frac{{m\left( {x^{2} + {\left( {m + 2} \right)x} + m + 1} \right)} - {{mx}\left( {{2x} + m + 2} \right)}}{\left( {x^{2} + {\left( {m + 2} \right)x} + m + 1} \right)^{2}}} \\{= \frac{{- {mx}^{2}} + m^{2} + m}{\left( {x^{2} + {\left( {m + 2} \right)x} + m + 1} \right)^{2}}}\end{matrix} & {{Equation}\mspace{14mu} 19}\end{matrix}$

The condition of maximizing the power transmission efficiency inEquation 19 is satisfied when x is expressed as following Equation 20.

x=√{square root over (m+1)}  Equation 20

If x in Equation 16 and m in Equation 17 are substituted into Equation20, following Equation 21 is obtained.

$\begin{matrix}{\frac{\omega \; Q_{3}L_{4}}{R_{L}} = \sqrt{{k_{2}^{2}Q_{2}Q_{3}} + 1}} & {{Equation}\mspace{14mu} 21}\end{matrix}$

When Equation 21 is arranged with respect to R_(L), following Equation22 is obtained.

$\begin{matrix}{R_{L} = \frac{\omega \; L_{4}Q_{3}}{\sqrt{{k_{2}^{2}Q_{2}Q_{3}} + 1}}} & {{Equation}\mspace{14mu} 22}\end{matrix}$

In other words, when the impedance R_(L) of the load 400 has the valuethe same as that of Equation 22, the power transmission efficiency ismaximized. In this case, the power transmission efficiency may becalculated as shown in FIG. 23.

$\begin{matrix}\begin{matrix}{{Efficiency} = \frac{mx}{x^{2} + {\left( {m + 2} \right)x} + m + 1}} \\{= \frac{m\sqrt{m + 1}}{m + 1 + {\left( {m + 2} \right)\sqrt{m + 1}} + m + 1}} \\{= \frac{m}{{2\sqrt{m + 1}} + \left( {m + 2} \right)}} \\{= \frac{m}{{2\sqrt{m + 1}} + \left( \sqrt{m + 1} \right)^{2} + 1}} \\{= \frac{m}{\left( {\sqrt{m + 1} + 1} \right)^{2}}} \\{= \frac{k_{2}^{2}Q_{2}Q_{3}}{\left( {\sqrt{{k_{2}^{2}Q_{2}Q_{3}} + 1} + 1} \right)^{2}}}\end{matrix} & {{Equation}\mspace{14mu} 23}\end{matrix}$

In other words, when the impedance R_(L) of the load 400 is the same asthat expressed as Equation 22, the maximum power transmission efficiencymay be obtained as Equation 23.

Referring to Equation 22, the impedance R_(L) of the load 400 to satisfythe condition of maximizing the power transmission efficiency may bevaried depending on the coupling coefficient K₂.

In detail, the relation between the coupling coefficient K₂ and theimpedance of the load 400 is shown as a graph in FIG. 4.

In FIG. 4, an x axis represents the coupling coefficient K₂ and a y axisrepresents a load impedance.

Referring to FIG. 4, as the coupling coefficient K₂ is increased, theload impedance is decreased. As the coupling coefficient K₂ isdecreased, the load impedance is increased. In other words, the powertransmission efficiency can be maximized when the load impedance isvaried depending on the coupling coefficient K₂. In detail, the powertransmission efficiency can be maximized when the load impedance isincreased as the coupling coefficient is increased and the loadimpedance is increased as the coupling coefficient is decreased.

The coupling coefficient K₂ may be varied depending on one of thedistance between the wireless power transmitter 200 and the wirelesspower receiver 300 and positions of the wireless power transmitter 200and the wireless power receiver 300 located in relation to each other.Accordingly, in order to obtain the maximum power transmissionefficiency, the impedance of the load 400 may be varied.

FIG. 5 is a graph showing the relation between the coupling coefficientK and the load impedance in detailed numeric values.

The load impedance 13.3Ω when the coupling coefficient K₂ is 0.05, theload impedance 8Ω when the coupling coefficient K₂ is 0.10, and the loadimpedance is 5Ω when the coupling coefficient K₂ is 0.25. Accordingly,the power transmission efficiency is maximized if the load impedance isreduced as the coupling coefficient K₂ is increased.

In general, the load 400 may include the battery of the cellular phone.The impedance of the battery may be varied depending on the quantity ofpower applied to the battery. In this case, for example, the load 400may include the battery of the cellular phone, but the embodiment is notlimited thereto. The load 400 may include various types of batteries ifthe impedance of the load 400 is varied depending on the quantity ofpower applied to the load 400.

FIG. 6 is a graph showing current as a function of voltage applied tothe battery.

The impedance R_(L) of the battery may be expressed as followingEquation 24.

$\begin{matrix}{R_{L} = \frac{V}{I}} & {{Equation}\mspace{14mu} 24}\end{matrix}$

In Equation 24, V represents voltage applied to the battery, and Irepresents current flowing through the battery.

If the voltage of 4 V is applied to the battery, the quantity of powerapplied to the battery is 1.2 W (4V×0.3 A). In this case, the impedanceof the battery becomes 13.3Ω (4V/0.3 A).

If the voltage of 4.583 V is applied to the battery, the quantity ofpower applied to the battery becomes 2.0 W (4.583V×0.437 A). In thiscase, the impedance of the battery becomes about 10.5Ω (4.458 V/0.437A).

If the voltage of 5V is applied to the battery, the quantity of powerapplied to the battery becomes 5.0 W (5V×1.0 A), and the impedance ofthe battery becomes 5.0Ω (5V/1 A).

In other words, as described above, the impedance of the battery may bevaried depending on the quantity of power applied to the battery.

In addition, when the relation between the load impedance and thequantity of power applied to the battery to satisfy the maximum powertransmission efficiency is represented as a graph based on the aboveresult, the graph is expressed as shown in FIG. 7.

In FIG. 7, an x axis represents the quantity of power applied to thebattery, and a y axis represents the impedance of the battery (load).

As shown in FIG. 7, the impedance of the battery may be varied dependingon the quantity of power applied to the battery.

In this case, when comparing FIG. 5 with FIG. 7, the graphs shown inFIGS. 5 and 7 are similar to each other. In detail, referring to FIG. 5,the impedance of the load is decreased as the coupling coefficient K₂ isincreased, and the impedance of the load is increased as the couplingcoefficient K₂ is decreased. Referring to FIG. 7, the impedance of thebattery is decreased as the quantity of power applied to the battery isincreased, and the impedance of the battery is increased as the quantityof power applied to the battery is decreased. The waveforms of thegraphs shown in FIGS. 5 and 7 are very similar to each other.

In other words, if the wireless power transmission system 10 employs theload 400 such as a battery having impedance varying depending on thequantity of power applied to the load 400, a specific correspondingrelation is established between the coupling coefficient K₂ and thereception power of the load 400. In this case, if the transmission poweris adjusted to establish the specific corresponding relation between thecoupling coefficient K₂ and the reception power of the load 400, thecondition to obtain the maximum power transmission efficiency shown inFIG. 5 can be satisfied.

In other words, the load impedance must be adjusted in order to obtainthe maximum transmission efficiency depending on the couplingcoefficient K₂. The adjustment of the load impedance is possible bycontrolling the quantity of power as shown in FIG. 7. In other words, ifthe reception power of the battery may be decided depending on thecoupling coefficient K₂, and the transmission power is adjusted suchthat the battery receives the decided reception power, the condition ofmaximizing the power transmission efficiency of FIG. 5 is satisfied, sothat the maximum power transmission efficiency can be obtained.

The corresponding relation may be represented as shown in the graph ofFIG. 8.

Referring to FIG. 8, the reception power received in the battery as afunction of the coupling coefficient K is shown as a graph. If thequantity of power received in the battery is 1.2 W when the couplingcoefficient is 0.05, the quantity of power received in the battery is2.0 W when the coupling coefficient K₂ is 0.10, and the quantity ofpower received in the battery is 5 W when the coupling coefficient K₂ is0.25, the condition to obtain the maximum power transmission efficiencyshown in FIG. 5 is satisfied.

Finally, in order to obtain the maximum power transmission efficiency,the reception power that must be sent to the load 400 must be decideddepending on the coupling coefficient K₂.

According to one embodiment, the wireless power transmitter 200 mayfurther include a storage unit (not shown) to store the reception powercorresponding to the coupling coefficient K₂. The wireless powertransmitter 200 searches the storage unit for the reception powercorresponding to the coupling coefficient K₂ and decide the receptionpower.

Referring to FIG. 3, the wireless power transmitter 200 determinespresent reception power received by the load 400 (step S107). Since thewireless power receiver 300 may send the power received from thewireless power transmitter 200 to the load 400 without power loss, thepower received by the wireless power receiver 300 is assumed as beingequal to the power received by the load 400.

The wireless power transmitter 200 may determine the present receptionpower received by the load 400 through various schemes.

According to one embodiment, the wireless power transmitter 200 maydetermine the present reception power received by the load 400 throughthe out-of-band communication described in FIG. 2. In detail, thewireless power transmitter 200 requests the information of the presentreception information received by the wireless power receiver 300through the out-of-band communication and receives the response to therequest, thereby determining the present reception power.

According to one embodiment, the wireless power transmitter 200 maydetermine the present reception power received in the load 400 bymeasuring the intensity of current flowing in the wireless powertransmitter 200. In this case, the wireless power transmitter 200 mayinclude the power supply device 100 described in FIG. 1. For example,the intensity of current flowing inside the wireless power transmitter200 may be related to the present reception power received by the load400. In detail, when the distance between the wireless power transmitter200 and the wireless power receiver 300 is constant, the intensity ofcurrent flowing in the wireless power transmitter 200 may be increasedas the quantity of power received by the load 400 is increased, and theintensity of current flowing inside the wireless power transmitter 200may be decreased as the quantity of the power received by the load 400is decreased.

The wireless power transmitter 200 may include a storage unit 170 tostore the intensity of current flowing inside the wireless powertransmitter 200 and the power received by the load 400 corresponding toeach other. The wireless power transmitter 200 may find the receptionpower corresponding to the intensity of current by searching for thestorage unit 170 and determine the present reception power received bythe load 400.

Thereafter, the wireless power transmitter 200 determines if thedetermined reception power is equal to the decided reception power (stepS109).

If it is determined that the determined reception power is differentfrom the decided reception power, the wireless power transmitter 200decides transmission power to be transmitted to the wireless powerreceiver 300 (step S111). In other words, the wireless power transmitter200 may decide transmission power corresponding to the decided receptionpower in order to obtain the maximum power transmission efficiency.

The wireless power transmitter 200 controls transmission power to betransmitted to the wireless power receiver 300 in order to transmit thedecided transmission power to the wireless power receiver 300 (stepS113). According to one embodiment, the wireless power transmitter 200may control the transmission power by controlling the power supply unit110 to supply the power to the power supply device 100, and the detailsthereof will be described in detail with reference to FIGS. 9 and 10.

According to still another embodiment, the wireless power transmitter200 may control the transmission power by measuring the current flowinginside the wireless power transmitter 200, and the details thereof willbe described with reference to FIGS. 11 and 12.

The wireless power transmitter 200 may receive the decided transmissionpower from the power supply device 100 and transmit the transmissionpower to the wireless power receiver 300. Accordingly, the load 400 mayreceive the reception power to satisfy the maximum power transmissionefficiency from the wireless power receiver 300.

As described above, according to the embodiment, the wireless powertransmitter 200 transmits the transmission power to maximize the powertransmission efficiency, and the load 400 may receive the receptionpower to make the power transmission efficiency maximized, so that thepower transmission efficiency can be maximized.

Hereinafter, a method of controlling power according to anotherembodiment will be described with reference to FIGS. 9 and 10 byincorporating the description made with reference to FIGS. 1 to 8. Inparticular, FIGS. 9 and 10 show a scheme of controlling the transmissionpower in step S113 of FIG. 3.

FIG. 9 is a block diagram showing the structure of a wireless powertransmission system according to another embodiment. FIG. 10 is a ladderdiagram to explain a method of controlling power according to anotherembodiment.

The wireless power transmission system 20 may include a power supplydevice 500, a wireless power transmitter 900, and the wireless powerreceiver 300.

The wireless power receiver 300 has the same components and structure asthose described with reference to FIGS. 1 to 2.

The wireless power transmitter 900 may receive DC power from the powersupply device 500. In detail, the wireless power transmitter 900transmits a voltage control signal to the power supply device 500 toreceive adjusted DC voltage.

The wireless power transmitter 900 may further include a transmissionunit 910, a power transmission state detecting unit 930, an oscillator940, an AC power generating unit 950, a control unit 960, a storage unit970, and a DC cut-off unit 980.

The power transmission state detecting unit 930 may detect the powertransmission state between the wireless power transmitter 900 and thewireless power receiver 300. According to one embodiment, the powertransmission state detecting unit 930 may detect the power transmissionstate based on the current flowing inside the power supply device 100.To this end, the power transmission state detecting unit 930 may use acurrent sensor. The current sensor may detect the current flowingthrough a circuit and measure the intensity of the detected current whenthe DC voltage received from the power supply device 500 is applied tothe AC power generating unit 950. However, a measurement point of thepower transmission state detecting unit 930 is not limited thereto, butmay include an output point of the AC power generating unit 950 to bedescribed later.

The intensity of current flowing inside the wireless power transmitter900 may be varied depending on the power transmission state between thewireless power transmitter 900 and the wireless power receiver 300.Details of the power transmission state will be described later.

According to one embodiment, the power transmission state detecting unit930 may include a current transformer (CT).

The oscillator 940 may generate an AC signal having a predeterminedfrequency. When the transmission unit 910 to be described latertransmits power to the wireless power receiver 300 through resonance,the oscillator 940 may generate an AC signal having a resonancefrequency to allow the transmission resonant coil included in thetransmission unit 910 to operate at the resonance frequency and transmitthe AC signal to the AC power generating unit 950. The AC signalgenerated from the oscillator 940 is applied to the AC power generatingunit 950.

The AC power generating unit 950 may generate AC power by using DC powerreceived from an AC-DC converter 510 of the power supply device 500based on the AC signal received from the oscillator 940.

The AC power generating unit 950 may amplify the AC signal received fromthe oscillator 940. According to one embodiment, the amplificationdegree of the AC signal may be varied depending on the intensity of theDC voltage applied to the AC power generating unit 950.

According to one embodiment, the AC power generating unit 950 mayinclude a push-pull type dual MOSFET.

The control unit 960 may control the overall operation of the wirelesspower transmitter 900.

The control unit 960 may detect the power transmission state variationbetween the wireless power transmitter 900 and the wireless powerreceiver 300. The control unit 960 may detect the power transmissionstate variation to decide the DC power to be received from the powersupply device 500, and may transmit a power control signal to the powersupply device 500 in order to receive the decided DC power through a PLCscheme. According to one embodiment, the power transmission state mayrelate to the distance between the wireless power transmitter 900 andthe wireless power receiver 300 and the directions in which the wirelesspower transmitter 900 and the wireless power receiver 300 are located.

According to one embodiment, the power transmission state may relate toa power reception state of the wireless power receiver 300. For example,if power charged in the wireless power receiver 300 is less than areference quantity of power, the wireless power receiver 300 may requestthe wireless power transmitter 900 to transmit power greater thanpresent power, which is being transmitted, through out-of-bandcommunication. However, the wireless power transmitter 900 may decidetransmission power to be transmitted to the wireless power receiver 300corresponding to the request. The wireless power transmitter 900 maydetermine the DC power to be received from the power supply device 500corresponding to the decided transmission power, and may control thepower supply device 500 in order to receive the determined DC power.Thereafter, the wireless power transmitter 900 may receive the decidedDC power from the power supply device 500 and convert the DC power intoAC power to be transmitted to the wireless power receiver 300.

The control unit 960 may detect the coupling state between the wirelesspower transmitter 900 and the wireless power receiver 300 by receivingthe information of the power transmission state through the powertransmission state detecting unit 930. According to one embodiment, ifthe power transmission state detecting unit 930 is a current sensor, thecontrol unit 960 may receive the intensity of current by the currentsensor and detect the distance between the wireless power transmitter900 and the wireless power receiver based on the intensity of thecurrent.

The control unit 960 may decide the DC voltage to be received from thepower supply device by using the detected distance. The control unit 960may transmit the voltage control signal including the information of thedecided DC voltage to the power supply device 500. In this case, thevoltage control signal may be transmitted between the wireless powertransmitter 900 and the power supply device 500 through the PLC scheme.The PLC scheme is a technology of carrying data on a high frequencysignal of several hundreds kHz to several tens MHz by employing a powerline to supply power as a medium. In other words, the PLC scheme may beperformed through a power line subject to a wiring work withoutseparately installing a dedicated communication line.

According to one embodiment, the control unit 960 may determine thedistance between the wireless power transmitter 900 and the wirelesspower receiver 300 based on the intensity of current.

According to one embodiment, the control unit 960 may decide DC voltageto be received from the power supply 500 based on the intensity of thecurrent instead of the determined distance.

The storage unit 970 may store the intensity of current measured by thecurrent sensor of the power transmission state detecting unit 930 andthe distance between the wireless power transmitter 900 and the wirelesspower receiver 300 corresponding to each other in the form of a look-uptable.

The storage unit 970 may store the intensity of current measured by thecurrent sensor of the power transmission state detecting unit 930 and DCvoltage to be received from the power supply device 500 by the wirelesspower transmitter 900 corresponding to each other in the form of alook-up table.

The storage unit 970 may store the intensity of current in the currentsensor of the power transmission state detecting unit 930, the distancebetween the wireless power transmitter 900 and the wireless powerreceiver 300, and the DC voltage to be received from the power supplydevice 500 by the wireless power transmitter 900 corresponding to eachother in the form of a look-up table.

The DC cut-off unit 980 may cut off a DC signal applied to the controlunit 960. According to one embodiment, the DC-cut off unit 980 mayinclude a capacitor.

The transmission unit 910 may wirelessly transmit AC power output fromthe AC power generating unit 950 to the wireless power receiver 300.

The power supply device 500 may include the AC-DC converter 510, thecontrol unit 520, and the DC cut-off unit 530. According to oneembodiment, the power supply device 500 may include an adaptor toconvert AC power received from an external device into DC power.

The AC-DC converter 510 may convert AC voltage received from theexternal device into DC voltage having a predetermined size. In thiscase, the AC voltage received from the outside may have the intensity of220V and the frequency of 60 Hz, but the embodiment is not limitedthereto. The control unit 520 receives the voltage control signal fromthe wireless power transmitter 900 to control the AC-DC converter 510 tooutput the DC voltage decided by the wireless power transmitter 900. Inother words, the control unit 520 may generate a voltage control signalto control the AC-DC converter 510 so that DC voltage is outputcorresponding to the intensity of current measured by the wireless powertransmitter 900. In this case, the AC-DC converter 510 may convert theAC voltage received from an outside into DC voltage having apredetermined size by receiving a voltage control signal and output theDC voltage.

The DC cut-off unit 530 may cut off the DC signal applied to the controlunit 520. According to one embodiment, the DC cut-off unit 530 mayinclude a capacitor.

FIG. 10 is a ladder diagram to explain a method of controlling poweraccording to another embodiment.

Hereinafter, the method of controlling the power according to anotherembodiment will be described by incorporating the description of FIG. 9.

Referring to FIG. 10, the current sensor of the power transmission statedetecting unit 930 may measure the intensity of current flowing throughinside the wireless power transmitter 900 (step S201). The currentsensor of the power transmission state detecting unit 930 may measurethe intensity of detected current by detecting the current flowinginside the wireless power transmitter 900.

According to one embodiment, the current sensor of the powertransmission state detecting unit 930 may measure the intensity ofcurrent input into the AC power generating unit 950 shown in FIG. 9. Inaddition, according to another embodiment, although the current sensorof the power transmission state detecting unit 930 may measure theintensity of current output from the AC power generating unit 950, theembodiment is not limited thereto. In other words, the current sensormay measure the intensity of current flowing inside the wireless powertransmitter 900.

According to one embodiment, the current sensor of the powertransmission state detecting unit 930 may include a CT. The CT maymeasure higher-intensity current flowing through the circuit by loweringthe higher-intensity current to lower-intensity current. In other words,the CT may measure current flowing through the circuit by transformingthe current flowing through the circuit into current proportional to thecurrent flowing through the circuit. In more detail, the CT may includea primary winding, a secondary winding, and an iron core. If theelectromagnetic induction phenomenon occurs due to magnetic flux passingthrough the iron core, the primary current may be transformed into thesecondary current in proportion to a CT ratio, and may measure thetransformed secondary current.

According to one embodiment, the current sensor of the powertransmission state detecting unit 930 may include one of a wound-typeCT, a bar-type CT, a through-type CT, a tertiary winding CT, and amulti-core CT.

According to one embodiment, the intensity of current measured by thecurrent sensor of the power transmission state detecting unit 930 may bevaried depending on the distance between the wireless power transmitter900 and the wireless power receiver 300. In other words, the increase inthe intensity of the current measured by the current sensor of the powertransmission state detecting unit 930 refers to that the wireless powertransmitter 900 is closer to the wireless power receiver. The decreasein the intensity of the current measured by the current sensor of thepower transmission state detecting unit 930 refers to that the wirelesspower transmitter 900 is gradually away from the wireless powerreceiver.

The distance between the wireless power transmitter 900 and the wirelesspower receiver 300 may refer to the distance between coils included inthe device.

The control unit 960 may determine the distance between the wirelesspower transmitter 900 and the wireless power receiver 300 based on theintensity of the measured current (step S203). The wireless powertransmitter 900 may determine the distance between the wireless powertransmitter 900 and the wireless power receiver 300 based on theintensity of the current measured through the control unit 960.According to one embodiment, the storage unit 970 may store theintensity of the measured current and the distance corresponding to eachother in the form of a look-up table, and the control unit 960 maydetermine the distance corresponding to the intensity of the measuredcurrent by searching for the storage unit 970.

The control unit 960 decides DC voltage to be received from the powersupply device 500 based on the determined distance (step S205).

According to one embodiment, the control unit 960 may decide DC voltageto be received from the power supply device 500 based on the intensityof the measured current instead of the distance. In this case, the stepS203 may be omitted. In other words, if the storage unit 970 stores theintensity of the measured current and the DC voltage to be received bythe wireless power transmitter 900 corresponding to each other, thecontrol unit 960 may decide DC voltage to be received by the wirelesspower transmitter 900 corresponding to the intensity of the measuredcurrent by searching for the storage unit 970.

According to still another embodiment, the storage unit 970 may storethe intensity of current measured by the power transmission statedetecting unit 930, the distance between the wireless power transmitter900 and the wireless power receiver 300, and the DC voltage to bereceived from the power supply device 500.

The wireless power transmitter 900 transmits the voltage control signalbased on the decided DC voltage to the power supply device 500 (stepS207). According to one embodiment, the voltage control signal may be asignal to control the power supply device 500 so that the wireless powertransmitter 900 may receive the decided DC voltage from the power supplydevice 500.

According to one embodiment, the wireless power transmitter 900 may makecommunication with the power supply device 500 through a PLC scheme. Thewireless power transmitter 900 may transmit the voltage control signalto the power supply device 500 through the PLC scheme. The PLC scheme isa technology of carrying data on a high frequency signal of severalhundreds kHz to several tens MHz by employing a power line to supplypower as a medium. In other words, the PLC scheme may be performedthrough a power line subject to a wiring work without separatelyinstalling a dedicated communication line.

As described above, when the voltage control signal is transmittedthrough the PLC scheme according to the embodiment, an additional powerline to transmit the voltage control signal is not required, so that thecost can be saved. In other words, according to the embodiment, sincethe voltage control signal is transceived by using the power lineserving as a medium to transmit power between the power supply device500 and the wireless power transmitter 900, an additional power line isnot required.

In addition, according to the embodiment, since the DC voltage receivedfrom the power supply device 500 may be adjusted through the PLC schemewithout the DC-DC converter to convert the DC voltage into predeterminedvoltage, the manufacturing cost of the wireless power transmitter 900may be greatly saved.

During the procedure in which the wireless power transmitter 900transmits the voltage control signal to the power supply device 500, theDC-cut off unit 980 may cut off a DC signal applied to the control unit960. The wireless power transmitter 900 receives the DC voltage from thepower supply device 500. If the DC voltage is applied to the controlunit 960, since the control unit 960 may be damaged. Accordingly, theDC-cut off unit 980 cuts off the DC voltage to protect the control unit960.

According to one embodiment, the DC-cut off unit 980 may include acapacitor. The impedance of the capacitor may be expressed as Xc=1/2πfC.If the DC signal is applied to the capacitor (frequency f=0), theimpedance becomes infinite to cut off the DC signal.

Since the voltage control signal transmitted to the power supply device500 by the wireless power transmitter 900 is an AC signal, the controlunit 960 may transmit the voltage control signal to the power supplydevice 500 regardless of the DC-cut off unit 980.

The power supply device 500 receives the voltage control signal from thewireless power transmitter 900 and generates a voltage control signal tooutput DC voltage to be transmitted to the wireless power transmitter900 according to the received voltage control signal (step S209). Thepower supply device 500 may generate the voltage control signal tooutput the DC voltage to be transmitted to the wireless powertransmitter 900 through the control unit 520. The control unit 520 maytransmit the voltage control signal to the AC-DC converter 510.

During the procedure in which the power supply device 500 receives thevoltage control signal from the wireless power transmitter 900, theDC-cut off unit 530 of the power supply device 500 may cut off a DCsignal applied to the control unit 960. The AC-DC converter 510 of thepower supply device 500 transmits the DC voltage to the wireless powertransmitter 900. If the DC voltage is applied to the control unit 520,the control unit 520 may be damaged. Accordingly, the DC cut-off unit530 may cut of the DC voltage to protect the control unit 520.

According to one embodiment, the DC-cut off unit 530 may include acapacitor. The impedance of the capacitor may be expressed as Xc=1/2πfC.If the DC signal is applied to the capacitor (frequency f=0), theimpedance becomes infinite to cut off the DC signal.

The power supply device 500 adjusts the DC voltage by receiving thevoltage control signal from the control unit 520 (step S211). The powersupply device 500 may adjust the DC voltage to be transmitted to thewireless power transmitter 200 by receiving the voltage control signalthrough the AC-DC converter 510. The AC-DC converter 510 may convert theAC voltage, which is applied from an outside, into predetermined DCvoltage based on the voltage control signal and output the AC voltage.

The power supply device 500 transmits the adjusted DC voltage to thewireless power transmitter 200 (step S213). The power supply device 500may transmit the DC voltage adjusted through the AC-DC converter 510 tothe wireless power transmitter 900.

The AC power generating unit 950 converts the received DC power into DCpower based on an AC signal having a predetermined frequency receivedfrom the oscillator 940 (step S215).

The AC power generating unit 950 transmits the output AC power to thetransmission unit 910 (step S217).

The AC power received in the transmission unit 910 may be transmitted tothe wireless power receiver 300 by resonance.

Since the power supplied from the power supply device may be controlleddepending on power transmission environments between the wireless powertransmitter and the wireless power receiver according to the embodimentas described above, an additional DC-DC converter is not required.Accordingly, the manufacturing cost of the wireless power transmittercan be significantly saved.

Hereinafter, a method of controlling power according to anotherembodiment will be described with reference to FIGS. 11 and 12 byincorporating the description made with reference to FIGS. 1 to 8. Inparticular, FIGS. 11 and 12 show a scheme of controlling thetransmission power in step S113 of FIG. 3.

FIG. 11 is a flowchart to explain a method of controlling poweraccording to another embodiment. FIG. 12 is a view to explain a look-uptable in which a current value when a first output voltage is applied toan AC power generating unit, a coupling coefficient, a second outputvoltage, and a preferable current range correspond to each other.

The description of the wireless power transmitter 200 is the same asthat of FIG. 1. In this case, the wireless power transmitter 200 mayinclude all components of the power supply device 100.

First, the control unit 180 controls the DC-DC converter 130 so that thevoltage applied to the AC power generating unit 160 is adjusted to afirst output voltage (step S301). In this case, the first output voltagemay refer to preset DC voltage.

Thereafter, the current sensor of the power transmission state detectingunit 140 may measure the intensity of current applied to the AC powergenerating unit 160 when the DC voltage output from the DC-DC converter130 is applied to the AC power generating unit 160 (step S303). Theintensity of the current applied to the AC power generating unit 160 maybe varied depending on the power transmission state between the wirelesspower transmitter 200 and the wireless power receiver 300. According toone embodiment, the power transmission state may refer to the distancebetween the wireless power transmitter 200 and the wireless powerreceiver 300, and the directions of the wireless power transmitter 200and the wireless power receiver 300. In other words, the powertransmission state may refer to the coupling state between the wirelesspower transmitter 200 and the wireless power receiver 300.

According to the present invention, the coupling state may becollectively referred to as an index related to the coupling coefficientbetween a transmission coil and a reception coil due to the distancebetween the wireless power transmitter 200 and the wireless powerreceiver 300 and the position relation between the wireless powertransmitter 200 and the wireless power receiver 300. In other words, thecoupling state according to the present invention may be collectivelyreferred to as all indexes related to a coupling coefficient such as thequantity of current flowing through the wireless power transmitter 200and the input impedance of the wireless power transmitter 200.

According to one embodiment, the power transmission state may refer toinformation of the power reception state of the wireless power receiver300.

In addition, the intensity of current applied to the AC power generatingunit 160 may be related to the coupling coefficient between thetransmission resonant coil unit 212 of the wireless power transmitter200 and the reception resonant coil unit 311. The coupling coefficientrefers to the degree of the electromagnetic coupling between thetransmission resonant coil unit 212 and the reception resonant coil unit311, and has the range of 0 to 1.

Meanwhile, the control unit 180 determines if the intensity of themeasured current is equal to or greater than a threshold value (stepS305). According to one embodiment, the threshold value may be 100 mAfor the illustrative purpose. The threshold value may refer to theminimum current value required to detect the wireless power receiver300. In other words, if the intensity of the measured current is equalto or greater than the threshold value, the wireless power receiver 300is regarded as being detected. If the intensity of the measured currentis less than the threshold value, the wireless power receiver 300 isregarded as not being detected.

If the intensity of the measured current is equal to or greater than thethreshold value, the control unit 180 decides second output voltagecorresponding to the intensity of the measured current (step S307). Thecontrol unit 180 may determine the second output voltage by searchingfor the DC voltage corresponding to the intensity of current applied tothe AC power generating unit 160 in the storage unit 170. According toone embodiment, the second output voltage may refer to voltage requiredto transmit power to the wireless power receiver 300.

Thereafter, the control unit 180 controls the DC-DC converter 130 toapply the decided second output voltage to the AC power generating unit160 (step S309). The DC-DC converter 130 outputs the second outputvoltage under the control of the control unit 180 and transmits thesecond output voltage to the AC power generating unit 160.

Thereafter, the current sensor 270 measures the intensity of currentapplied to the AC power generating unit 160 again (step S311).

Thereafter, the control unit 180 may determine if the intensity of themeasured current is in a preferable current range (step S313). In thiscase, the preferable current range may refer to a current rangecorresponding to the second output voltage when the second outputvoltage is applied to the AC power generating unit 160. The preferablecurrent range may be increased as the second output voltage isincreased. As the second output voltage is decreased, the range of thesecond output voltage may be decreased.

The control unit 180 may search the storage unit 170 for the preferablecurrent range corresponding to the second output voltage, and maydetermine if the intensity of the measured current is in the preferablecurrent range.

If the intensity of the measured current is in the preferably currentrange, the control unit 180 stands by for a predetermined time (stepS315) and returns to step S311. In other words, the control unit 180 maymeasure the intensity of current applied to the AC power generating unit160 and periodically determine if the intensity of the measured currentcorresponds to the second output voltage applied to the AC powergenerating unit 160.

Meanwhile, if the intensity of the current measured in step S105 is lessthan the threshold value, the control unit 180 controls the DC-DCconverter 130 to adjust the first output voltage to 0V (step S317).

In other words, if the intensity of the measured current is less thanthe threshold value, the control unit 180 determines that the wirelesspower receiver 300 is not detected and thus adjusts the first outputvoltage to 0V. If the voltage applied to the AC power generating unit160 is 0V, the wireless power transmitter 200 does not transmit power tothe wireless power receiver 300.

Therefore, if the wireless power receiver 300 is not detected, thewireless power transmitter 200 can inhibit meaningless power loss.

Meanwhile, if the first output voltage is adjusted to 0V, the controlunit 180 stands by for 0.1 second (step S319). In this case, 0.1 secondis provided for the illustrative purpose.

If 0.1 second elapses, the control unit 180 returns to step S301 so thatthe DC voltage applied to the AC power generating unit 160 is adjustedto the first output voltage.

Meanwhile, if the intensity of current measured in step S313 is not inthe preferable current range, the control unit 180 returns to step S307.In other words, the control unit 180 may control the DC-DC converter 130such that the second output voltage corresponding to the intensity ofthe current measured in step S113 is applied to the AC power generatingunit 160. The intensity of the current measured in step S313 may referto the power reception state of the wireless power receiver 300.

For example, if the intensity of the current measured in step S313 ismeasured less than the preferable current range, the distance betweenthe wireless power transmitter 200 and the wireless power receiver 300may be regarded as being shorter. Accordingly, the control unit 180 maycontrol the DC-DC converter 130 to apply DC voltage, which is morereduced by one level, to the AC power generating unit 160, therebyreducing the quantity of the transmission power transmitted to thewireless power receiver 300.

As described above, according to the method of controlling the poweraccording to the embodiment, the power reception state of the wirelesspower receiver 300 is detected based on the intensity of current appliedto the AC power generating unit 160 and the quantity of the transmissionpower may be more adjusted in order to transmit the power correspondingto the power reception state. Accordingly, the power transmissionefficiency can be maximized and the quantity of the power loss can bereduced.

FIG. 12 is a view to explain a look-up table in which a current valuemeasured when a first output voltage is applied to an AC powergenerating unit, a coupling coefficient, a second output voltage, and apreferable current range correspond to each other.

The look-up table of FIG. 12 is stored in the storage unit 17.

If current measured by the power transmission state detecting unit 140is equal to or greater than 100 mA when the first output voltage isapplied to the AC power generating unit 160, the wireless power receiver300 is regarded as being detected.

The first output voltage may be 12V for the illustrative purpose.

If the current measured by the power transmission state detecting unit140 is equal to or greater than 120 mA when the first output voltage isapplied to the AC power generating unit 160, the coupling coefficient ofthe transmission resonant coil unit 212 of the wireless powertransmitter 200 and the reception resonant coil unit 311 of the wirelesspower receiver 300 correspond to 0.05. In this case, the control unit180 determines the wireless power receiver 300 as being away from thewireless power transmitter 200, and controls the DC-DC converter 130 sothat the DC voltage applied to the AC power generating unit 160 becomes28V (second output voltage).

Thereafter, when the DC voltage applied to the AC power generating unit160 is maintained to 28V, the control unit 180 determines if the currentapplied to the AC power generating unit 160 satisfies the condition ofthe preferable current range of 751 mA to 800 mA.

If the current applied to the AC power generating unit 160 is beyond thepreferable current range, the first output voltage (12 V) is applied tothe AC power generating unit 160 for the measurement of current. If thevalue of the measured current is 180 mA, the control unit 180 determinesthe wireless power transmitter 200 as being closer to the wireless powerreceiver 300 when comparing with the case that the value of the measuredcurrent is 120 mA.

Although the distance between the wireless power transmitter 200 and thewireless power receiver 300 has been described in relation to theintensity of current in the above example, various power transmissionstates such as the directions in which the wireless power transmitter200 and the wireless power receiver 300 are located may be considered.

As described above, the wireless power transmitter 200 adjusts the powertransmitted to the wireless power receiver 300 by taking intoconsideration various power transmission states such as the distancefrom the wireless power receiver 300 and the direction in relation tothe wireless power receiver 300, thereby maximizing the powertransmission efficiency and inhibiting power loss.

Hereinafter, a scheme of detecting a coupling coefficient according toanother embodiment will be described with reference to FIGS. 13 to 15will be described by incorporating the description made with referenceto FIGS. 1 to 3.

FIG. 13 is a flowchart to explain the scheme of detecting a couplingcoefficient according to another embodiment. FIG. 14 is a view toexplain the case that a switch SW is open in order to change outputimpedance Z_(L). FIG. 15 is a view to explain the case that the switchSW is shorted in order to change the output impedance Z_(L).

Hereinafter, a scheme of detecting a coupling coefficient according toanother embodiment will be described with reference to FIG. 13.

First, the wireless power receiver 300 changes an output impedance (stepS401). The output impedance Z_(L) may refer to an impedance measuredwhen viewed the load 400 from the reception unit 310. The wireless powerreceiver 300 may include the switch SW, and may change the outputimpedance through the switch SW. One terminal of the switch SW isconnected to a capacitor C₄, and an opposite terminal of the switch SWis connected to one terminal of the load 400. An opposite terminal ofthe capacitor C₄ is connected to the one terminal of the load 400.

Referring to FIG. 14, the wireless power receiver 300 transmits an opensignal to the switch SW to open the switch SW. If the switch SW is open,the output impedance Z_(L) may be expresses as Equation 25.

$\begin{matrix}{Z_{L} = {R_{L} + \frac{1}{{j\omega}\; C_{4}}}} & {{Equation}\mspace{14mu} 25}\end{matrix}$

If resistors R₂ and R₃ are decided to 0Ω on the assumption that theresistors R₂ and R₃ have very small values in Equation 1, Equation 3,and Equation 5, and the values of the transmission induction coil L₁ andthe capacitor C₁, the transmission resonant coil L₂ and the capacitorC₂, the reception resonant coil L₃ and capacitor C₃, and the receptioninduction coil L₄ and the capacitor C₄ are set in such a manner that allof the above coils and the capacitors make resonance at the resonancefrequency w, the first input impedance Z1 in Equation 5 may be arrangedas Equation 26.

$\begin{matrix}{Z_{1} = {\frac{M_{1}^{2}M_{3}^{2}}{M_{2}^{2}}\frac{\omega^{2}}{Z_{L} + {{j\omega}\; L_{4}}}}} & {{Equation}\mspace{14mu} 26}\end{matrix}$

Equation 26 may be arranged as following Equation 27 by using Equation2, Equation 4, and Equation 6.

$\begin{matrix}{Z_{1} = {\frac{k_{1}^{2}k_{3}^{2}}{k_{2}^{2}}\frac{\omega^{2}L_{1}L_{4}}{Z_{L} + {{j\omega}\; L_{4}}}}} & {{Equation}\mspace{14mu} 27}\end{matrix}$

If the values of the reception induction coil L₄ and the capacitor C₄are decided so that the reception induction coil L₄ and the capacitor C₄make resonance at the resonance frequency ω, and the output impedanceZ_(L) is substituted into Equation 27, the first input impedance Z₁ isarranged as following Equation 28.

$\begin{matrix}{Z_{1} = {\frac{k_{1}^{2}k_{3}^{2}}{k_{2}^{2}}\frac{\omega^{2}L_{1}L_{4}}{R_{L}}}} & {{Equation}\mspace{14mu} 28}\end{matrix}$

Referring to FIG. 15, the wireless power receiver 300 shorts the switchSW by transmitting a short signal. If the switch SW is shorted, theoutput impedance Z_(L) becomes 0, and the first input impedance Z₁ isarranged as Equation 29.

$\begin{matrix}{Z_{1} = {\frac{k_{1}^{2}k_{3}^{2}}{k_{2}^{2}}\left( {{- {j\omega}}\; L_{1}} \right)}} & {{Equation}\mspace{14mu} 29}\end{matrix}$

The wireless power receiver 300 may short the switch SW for apredetermined time at a predetermined period by applying the controlsignal to the switch SW. The period may be 1 second, and thepredetermined time may be 100 μs for the illustrative purpose.

Thereafter, the detection unit 220 measures the input impedance (StepS403). According to one embodiment, the detection unit 220 may measurethe first input impedance Z₁ by using current and voltage input to thewireless power transmitter 220 from the power supply device 100.

Thereafter, the detection unit 220 may detect the coupling coefficientbetween the transmission resonant coil L₂ of the transmission unit 210and the reception resonant coil L₃ of the reception unit 310 by usingthe measured input impedance (step S405). In other words, referring toEquation 29 and Equation 30, since all variables other than the couplingcoefficient K₂ have fixed values, the coupling coefficient K₂ may bedetected if the first input impedance Z₁ is measured.

Hereinafter, a method of controlling power according to still anotherembodiment will be described with reference to FIGS. 16 to 17.

FIG. 16 is a flowchart to explain the method of controlling poweraccording to still another embodiment. FIG. 17 is a view to explain alook-up table used in the method of controlling power according to theembodiment of FIG. 16.

Referring to FIG. 16, the wireless power transmitter 200 measures theinput impedance (step S501).

The detection unit 220 detects the coupling coefficient between thetransmission resonant coil unit 212 and the reception resonant coil unit311 by using the measured input impedance (step S503). Since the schemeof detecting the coupling coefficient is the same as that described withreference to FIGS. 3 and 13, the details of the scheme of detecting thecoupling coefficient is omitted.

The wireless power transmitter 200 searches for transmission powercorresponding to the detected coupling coefficient (step S505). Thestorage unit 170 of the wireless power transmitter 200 storestransmission power according to the coupling coefficient incorrespondence to the coupling coefficient in the form of a look-uptable. The wireless power transmitter 200 searches the storage unit 170for the transmission power corresponding to the detected couplingcoefficient.

The look-up table will be described with reference to FIG. 17.

Referring to FIG. 17, a look-up table in which a distance, input testcurrent, a coupling coefficient, load impedance, reception power, powertransmission efficiency and transmission power correspond to each othercan be shown.

In this case, the distance may refer to the distance between thewireless power transmitter 200 and the wireless power receiver 300. Indetail, the distance between the wireless power transmitter 200 and thewireless power receiver 300 may be the distance between the transmissionresonant coil unit 212 and the reception resonant coil unit 311 shown inFIG. 2. Referring to FIG. 17, as the distance between the wireless powertransmitter 200 and the wireless power receiver 300 is longer, thecoupling coefficient may be reduced.

The input test current is current applied to the wireless powertransmitter 200.

The power transmission efficiency may refer to the power transmissionefficiency between the wireless power transmitter 200 and the wirelesspower receiver 300, or the power transmission efficiency between thewireless power transmitter 200 and the load 400.

The load impedance may refer to impedance of the load 400 to obtain themaximum power transmission efficiency. It may be recognized that therelation between the load impedance and the coupling coefficient is thesame as the characteristic of the graph shown in FIG. 4.

The reception power is power received by the load 400. The receptionpower represents power that the load 400 must receive in order to obtainthe maximum power transmission efficiency.

The transmission power is power that must be transmitted from thewireless power transmitter 200 to the wireless power receiver 300 inorder to obtain the maximum power transmission efficiency.

The wireless power transmitter 200 may search the look-up table for thetransmission power corresponding to the detected coupling coefficient.

The wireless power transmitter 200 may decide the transmission powerobtained through the search as the transmission power to be transmittedto the load 400 (step S507). In other words, the wireless powertransmitter 200 may decide the transmission power corresponding to thedetected coupling coefficient to obtain the maximum power transmissionefficiency.

The wireless power transmitter 200 controls the transmission power inorder to transmit the decided transmission power to the load 400 (stepS509). According to one embodiment, the wireless power transmitter 200may use a scheme of controlling the transmission power by controllingthe power supply device 500 that supplies power to the power supplydevice 100, and the details thereof has been described with reference toFIGS. 9 and 10.

According to still another embodiment, the wireless power transmitter200 may control the transmission power by measuring current flowinginside the wireless power transmitter 200, and the details thereof havebeen described with reference to FIGS. 11 and 12.

According to a scheme of controlling power of still another embodiment,since the transmission power can be determined through the detection ofthe coupling coefficient and the search of the storage unit, theconfiguration of components is simple, and the procedure of controllingpower is simple.

The method of controlling power according to the embodiment may berealized in the form of a program executed in a computer and stored in acomputer-readable medium. The computer-readable recording mediumincludes a ROM, a RAM, a CD-ROM, a magnetic tape, a floppy disk, and anoptical data storage device. Further, the computer-readable recordingmedium may be implemented in the form of a carrier wave (for example,transmission through Internet).

The computer-readable recording medium may be distributed in computersystems connected with each other through a network and a code which isreadable by a computer in a distribution scheme may be stored andexecuted in the computer-readable recording medium. A functionalprogram, a code and code segments for implementing the method may beeasily deduced by programmers skilled in the related art.

What is claimed is:
 1. A wireless power transmitter comprising: a powersupply unit to supply AC power to the wireless power transmitter; and atransmission coil to transmit the AC power to a reception coil of awireless power receiver by resonance, wherein the wireless powertransmitter controls transmission power to be transmitted to thewireless power receiver based on a coupling state between thetransmission coil and the reception coil.
 2. The wireless powertransmitter of claim 1, wherein the wireless power transmitter decides afirst reception power to be received by a load according to the couplingstate and controls the transmission power such that the decided firstreception power is transmitted to the load.
 3. The wireless powertransmitter of claim 2, wherein the wireless power transmitterdetermines a second reception power, which is presently received by theload, and controls the transmission power such that the first receptionpower is transmitted to the load if the second reception power isdifferent from the first reception power.
 4. The wireless powertransmitter of claim 3, wherein the coupling state is a couplingcoefficient between the transmission coil and the reception coil, andthe wireless power transmitter determines the second reception power bydetecting the coupling coefficient.
 5. The wireless power transmitter ofclaim 4, wherein the wireless power transmitter determines the secondreception power through in-band communication or out-of-bandcommunication with the wireless power receiver.
 6. The wireless powertransmitter of claim 1, further comprising a detection unit to detect acoupling state between the wireless power transmitter and the wirelesspower receiver.
 7. The wireless power transmitter of claim 6, whereinthe coupling state is a coupling coefficient between the transmissioncoil and the reception coil, and the detection unit measures an inputimpedance obtained when viewed from the power supply device to thewireless power transmitter and detects the coupling coefficient based onthe measured input impedance.
 8. The wireless power transmitter of claim7, wherein the detection unit detects the coupling state by measuring aninput impedance of the wireless power transmitter after fixing an outputimpedance of the wireless power receiver.
 9. The wireless powertransmitter of claim 7, further comprising a storage unit to store thecoupling coefficient and the transmission power corresponding to eachother, wherein the wireless power transmitter searches for thetransmission power corresponding to the detected coupling coefficient totransmit the searched transmission power to the wireless power receiver.10. The wireless power transmitter of claim 1, wherein the power supplyunit receives DC power from a power supply unit to output the AC power,and the wireless power transmitter controls the transmission power byadjusting the AC power output from the power supply unit.
 11. Thewireless power transmitter of claim 10, wherein the power supply unittransmits a power control signal to control the DC power output from thepower supply unit through power line communication.
 12. The wirelesspower transmitter of claim 1, wherein the power supply unit comprises anAC power generating unit to generate the AC power by receiving the DCpower from the power supply unit, and the wireless power transmitterdetermines the coupling state based on an intensity of current input tothe AC power generating unit or output from the AC power generating unitand controls the transmission power according to the determined couplingstate.
 13. A method of controlling power of a wireless powertransmitter, the method comprising: detecting a coupling state betweenthe wireless power transmitter and a wireless power receiver; adjustingtransmission power based on the coupling state; and transmitting theadjusted transmission power to a load by resonance.
 14. The method ofclaim 13, wherein the adjusting of the transmission power comprises:determining a first reception power to be received by the loadcorresponding to the coupling state; and adjusting the transmissionpower such that the determined first reception power is transmitted tothe load.
 15. The method of claim 14, wherein the adjusting of thetransmission power such that the determined first reception power istransmitted to the load comprises, determining a second reception powerpresently received by the load, comparing the first reception power withthe second reception power and, adjusting the transmission power suchthat the first reception power is transmitted to the load if the firstreception power is different from the second reception power as acomparison result.
 16. The method of claim 15, wherein the determiningof the second reception power, which is presently received by the load,comprises determining the second reception power through in-bandcommunication or out-of-band communication with the wireless powerreceiver.
 17. The method of claim 15, wherein the determining of thesecond reception power presently received by the load comprisesdetermining the second reception power by measuring an intensity ofcurrent flowing inside the power supply device.
 18. The method of claim13, wherein the detecting of the coupling state comprises detecting acoupling coefficient between a transmission coil of the wireless powertransmitter and a reception coil of the wireless power receiver.
 19. Themethod of claim 18, wherein the adjusting of the transmission powercomprises determining the transmission power by searching a storage unitto store the coupling coefficient and the transmission powercorresponding to the coupling coefficient.
 20. The method of claim 13,wherein the detecting of the coupling state comprises detecting thecoupling state by using an input impedance of the wireless powertransmitter.